Japanese Patent Application No. 2004-231912, filed on Aug. 9, 2004, is hereby incorporated by reference in its entirety.
1. Technical Field
The present invention relates to a solid state imaging device and a driving method thereof.
2. Related Art
Solid state imaging devices that are mounted on mobile phones, digital cameras or the like, are generally provided with image sensors such as a CCD-type (Charge Coupled Device) image sensor (hereinafter called CCD sensor) or a CMOS-type image sensor (hereinafter called CMOS sensor).
In recent years, a MOS imaging device of a threshold voltage modulation system (hereinafter called modulation type MOS sensor), which combines high image quality and low power consumption, has been suggested. Japanese Unexamined Patent Publication No. 2002-134729 is a first example of related art for a modulation type MOS sensor.
CCD sensors require a high driving voltage, and hence have high power consumption, while they attains the Correlated Double Sampling (CDS) function for removing noise, and the synchronous electronic shutter function for photographing objects that move in high speed without distorting the object's image. This synchronous electronic shutter function eliminates the distortion of the object's image by simultaneously storing optically generated charges for multiple light receiving elements that are arrayed in two dimensions. Thus CCD sensors are generally provided with an advantage of higher quality.
In contrast, a specific type of CMOS sensor called CMOS-APS (Active Pixel Sensor) having a 4-transistors structure, attains the CDS function, while it does not attain the synchronous electronic shutter function. In general, CMOS sensors have the advantages of a low power consumption due to their low driving voltage and a low process cost. The reason, for which the synchronous electronic shutter function is not employed in common CMOS-APS type sensors, is that they are operated in order to achieve the CDS function. This function resets a floating diffusion (hereinafter FD) per every read-out line, where the FD retains an electric charge, first reading out a noise component, and subsequently reading out a signal component.
More specifically, in order to attain the CDS function in CMOS-APS sensors, transistors transferring electric charges are sequentially reset at every selected line from which an image signal is read out, first reading out the noise component, and subsequently reading out the signal component. The read-out of the signal component is performed while the transistors are sequentially reset per every selected line. Therefore, the read-out timings gradually deviate from the first read-out line to the last read-out line when objects moving in high speed are photographed, resulting in a distortion of the image obtained from the object.
While it is not impossible to materialize the synchronous electronic shutter function in CMOS-APS type sensors, the above-mentioned transfer transistors are also used for this function. Therefore, the CDS function cannot be attained together with the synchronous electronic shutter function in the CMOS-APS type sensors, involving a problem of image quality degradation.
In the modulation type MOS sensor that is disclosed in the first example of related art, the signal component is first read-out, and after the reset, the noise component is read out. Consequently, the difference between those two signal components is output as a pixel signal.
In the case of this modulation type MOS sensor, a noise component that remained after the previous reset is included in a signal component being read out, while a noise component that remained after the current reset is read out subsequently afterwards. There is no guarantee that the quantity of the noise component remaining from the previous reset, which is included in the signal component, and that of the noise component remaining from the current reset, are the same. In other words, the previous noise component, not the current noise component, is included in the output pixel signal. Therefore, in the case of the modulation type MOS sensor, the signal component and the noise component have not been correlated, and there has been a drawback that the noise is not removed accurately. This leads to degradation of the image quality.
Japanese Unexamined Patent Publication No. 2004-87963 is a second example of related art for modulation type MOS sensor, in which the technique for attaining the synchronous electronic shutter is suggested for modulation type MOS sensors. In case of the technique related to this suggestion, the entire pixel is reset at once, and thereafter the read-out of pixel signals is performed per every line.
Further, in case of the modulation type MOS sensor related to those suggestions, a ring gate is used for a modulation transistor. Each of the plurality of sensor cells arranged in a two dimensional matrix on a substrate have one ring gate for one photodiode.
However, in the technique according to the second example of the related art mentioned above, upon reading out a pixel signal, the signal component is first read out, and after the reset the noise component is subsequently read out. Hence the signal component and the noise component are not correlated, where the problem of the noise not being removed accurately still remains.
Moreover, in the case of the modulation type MOS sensor according to the first and the second example of the related art mentioned above, a distance between the ring gate of the modulation transistor and a drain, needs to be kept wider than a certain distance, in order to maintain the efficiency of modulation. Therefore, it involves a problem of not being able to make the modulation type MOS sensor microscopic, since in the modulation type MOS sensor, the width of the ring gate needs to be larger than a certain width.